Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

A bulging apparatus has a heating station, a tube expanding station, a
preforming station, and a main forming station. A straight tube as a
workpiece is gripped by first holding mechanisms or second holding
mechanisms, and delivered between the stations as the first holding
mechanisms or second holding mechanisms are displaced. During a
preforming process and a main holding process, both the first holding
mechanisms and second holding mechanisms can be lifted and lowered
vertically and can be moved horizontally.

Claims:

1. A bulging method for performing a forming process on a hollow member by
introducing a fluid under pressure thereinto, comprising the steps
of:gripping said hollow member at opposite ends thereof with rods, which
are movable back and forth, of first holding mechanisms, with fluid
pressure passages defined in the rods;introducing a fluid under pressure
into said hollow member gripped by said rods to expand said hollow
member, and stopping said hollow member with a tube expanding
die;displacing said first holding mechanisms from said tube expanding die
to a preforming die with displacing mechanisms to deliver the expanded
hollow member to said preforming die;preforming said hollow member with
said preforming die; andgripping said hollow member at the opposite ends
thereof with rods, which are movable back and forth, of second holding
mechanisms, with fluid pressure passages defined in the rods, displacing
said second holding mechanisms from said preforming die to a main forming
die with displacing mechanisms to deliver the preformed hollow member to
said main forming die; andforming said hollow member into a product shape
with said main forming die.

2. A bulging method according to claim 1, wherein a forming process is
performed on said hollow member by said main forming die after the
preformed hollow member is angularly moved.

3. A bulging method according to claim 1, wherein said hollow member is
supported so as to be translatable in either a vertical direction or a
horizontal direction when said hollow member is preformed or mainly
formed.

4. A bulging method for performing a forming process on a hollow member by
introducing a fluid under pressure thereinto, comprising the steps
of:gripping said hollow member at opposite ends thereof with rods, which
are movable back and forth, of holding mechanisms, with fluid pressure
passages defined in the rods;introducing a fluid under pressure into said
hollow member gripped by said rods to expand said hollow member, and
stopping said hollow member with a tube expanding die;displacing said
holding mechanisms from said tube expanding die to a main forming die
with displacing mechanisms to deliver the expanded hollow member to said
main forming die; andforming said hollow member into a product shape with
said main forming die.

5. A bulging method according to claim 4, further comprising the step of
preforming said hollow member with a preforming die which is disposed
between said tube expanding die and said main forming die.

6. A bulging method according to claim 5, wherein a forming process is
performed on said hollow member by said main forming die after the
preformed hollow member is angularly moved.

7. A bulging method according to claim 4, wherein said hollow member is
supported so as to be translatable in either a vertical direction or a
horizontal direction when said hollow member is preformed or mainly
formed.

8. A bulging apparatus for performing a forming process on a hollow member
by introducing a fluid under pressure thereinto, comprising:first holding
mechanisms and second holding mechanisms having rods for gripping said
hollow member at opposite ends thereof, the rods being movable back and
forth, with fluid pressure passages defined in the rods;a tube expanding
die for stopping said hollow member which is gripped by said rods, while
said hollow member is being expanded by a fluid under pressure being
introduced thereinto;a preforming die for preforming the expanded hollow
member;a main forming die for forming the preformed hollow member into a
product shape; anddisplacing mechanisms for displacing said first holding
mechanisms from said tube expanding die to said preforming die or from
said preforming die to said tube expanding die, and displacing said
second holding mechanisms from said preforming die to said main forming
die or from said main forming die to said preforming die;wherein said
first holding mechanisms and said second holding mechanisms are displaced
by said displacing mechanisms to deliver the hollow member gripped by the
rods of said first holding mechanisms from said tube expanding die to
said preforming die and to deliver the hollow member gripped by the rods
of said second holding mechanisms from said preforming die to said main
forming die.

9. A bulging apparatus according to claim 8, further comprising turning
mechanisms for angularly moving said first holding mechanisms and/or said
second holding mechanisms.

10. A bulging apparatus according to claim 8, further comprising a heating
unit for heating said hollow member to be delivered to said tube
expanding die.

11. A bulging apparatus according to claim 8, further comprising at least
one of lifting/lowering mechanisms for vertically translating said first
holding mechanisms and/or said second holding mechanisms, and translating
mechanisms for horizontally translating said first holding mechanisms
and/or said second holding mechanisms.

12. A bulging apparatus for performing a forming process on a hollow
member by introducing a fluid under pressure thereinto,
comprising:holding mechanisms having rods for gripping said hollow member
at opposite ends thereof, the rods being movable back and forth, with
fluid pressure passages defined in the rods;a tube expanding die for
stopping said hollow member which is gripped by said rods, while said
hollow member is being expanded by a fluid under pressure being
introduced thereinto;a main forming die for forming the expanded hollow
member into a product shape; anddisplacing mechanisms for displacing said
holding mechanisms from said tube expanding die to said main forming die
or from said main forming die to said tube expanding die;wherein said
holding mechanisms are displaced by said displacing mechanisms to deliver
the hollow member gripped by the rods from said tube expanding die to
said main forming die.

13. A bulging apparatus according to claim 12, further comprising a
preforming die disposed between said tube expanding die and said main
forming die, for preforming the expanded hollow member.

14. A bulging apparatus according to claim 13, further comprising turning
mechanisms for angularly moving said holding mechanisms.

15. A bulging apparatus according to claim 12, further comprising a
heating unit for heating said hollow member to be delivered to said tube
expanding die.

16. A bulging apparatus according to claim 12, further comprising at least
one of lifting/lowering mechanisms for vertically translating said
holding mechanisms, and translating mechanisms for horizontally
translating said holding mechanisms.

Description:

TECHNICAL FIELD

[0001]The present invention relates to a bulging method for performing a
forming process on a hollow member by introducing a fluid under pressure
thereinto, and an apparatus for carrying out such a bulging method.

BACKGROUND ART

[0002]Bulging processes have been employed to produce hollow formed bodies
which are long and whose cross-sectional shapes and dimensions
perpendicular to the longitudinal direction thereof differ from position
to position (see, for example, Patent Document 1). As described in Patent
Documents 2, 3, bulging apparatus for performing bulging processes have a
single compressing mechanism that is combined with interchangeable dies.
The bulging apparatus have a plurality of dies each interchangeable with
another die suitable for a shape to be formed.

[0003]A bulging process using a straight tube as a blank workpiece will
specifically be described below. First, the straight tube is gripped and
placed in a die. Then, a fluid under pressure (generally water under high
pressure) is supplied into the straight tube.

[0004]Therefore, the straight tube has its inner circumferential wall
pressed by the fluid under pressure, and is expanded diametrically
outwardly. As the straight tube is placed in the die, the expanded
portion of the straight tube is finally stopped by the die. Therefore,
the straight tube is formed into a shape corresponding to the cavity of
the die. This process is also referred to as a tube expanding process.

[0005]Then, the die is removed from the compressing mechanism, and another
die is mounted in the compressing mechanism for performing a next forming
process. At this time, the die used in the tube expanding process is
retracted from the compressing mechanism, and the die to be used in the
forming process is moved to the compressing mechanism.

[0006]The expanded straight tube is transferred to the die, and then
compressed to a predetermined shape by the compressing mechanism. A final
formed product is now obtained.

[0010]As can be understood from the foregoing, it has heretofore been
customary to move dies to replace one with the other in the bulging
process. However, since it takes a long time to move the heavy dies, the
cycle time for producing a final formed product from a straight tube is
long.

[0011]Furthermore, it is not easy to form a complexly shaped product such
as a frame for an automobile body or the like only in two processes,
i.e., the tube expanding process and the final forming process. It has
been considered to perform a preforming process after the tube expanding
process. However, using three interchangeable dies on one compressing
mechanism tends to make the apparatus complicated in structure. It is not
easy to install the three dies movably on the compressing mechanism. Even
if the three dies are installed movably on the compressing mechanism, a
wide space is required.

[0012]In order to avoid the above drawbacks, it may be proposed to prepare
individually a die for performing the tube expanding process, a die for
performing the preforming process, and a die for performing the finally
forming process, and to deliver a workpiece to the dies with a robot.
However, since the robot is needed, the apparatus is complex in structure
and the investment for facilities is high.

[0013]It is a general object of the present invention to provide a bulging
process which is capable of shortening a cycle time.

[0014]A major object of the present invention is to provide a bulging
apparatus which is simple in structure even though it has two or more
dies.

[0015]Another object of the present invention is to provide a bulging
apparatus which makes it difficult for a hollow member being formed to be
deformed.

[0016]According to an aspect of the present invention, a bulging method
for performing a forming process on a hollow member by introducing a
fluid under pressure thereinto, comprises the steps of:

[0017]gripping the hollow member at opposite ends thereof with rods, which
are movable back and forth, of first holding mechanisms, with fluid
pressure passages defined in the rods;

[0018]introducing a fluid under pressure into the hollow member gripped by
the rods to expand the hollow member, and stopping the hollow member with
a tube expanding die;

[0019]displacing the first holding mechanisms from the tube expanding die
to a preforming die with displacing mechanisms to deliver the expanded
hollow member to the preforming die;

[0020]preforming the hollow member with the preforming die; and

[0021]gripping the hollow member at the opposite ends thereof with rods,
which are movable back and forth, of second holding mechanisms, with
fluid pressure passages defined in the rods, displacing the second
holding mechanisms from the preforming die to a main forming die with
displacing mechanisms to deliver the preformed hollow member to the main
forming die; and

[0022]forming the hollow member into a product shape with the main forming
die.

[0023]According to the present invention, the hollow member (workpiece)
gripped by the holding mechanisms is delivered between the dies. Stated
otherwise, not the dies which are heavy, but the workpiece is delivered
together with the holding mechanisms. Since the workpiece is much smaller
and lighter than the dies, it can be delivered with utmost ease.
Therefore, the cycle time required until a final formed product is
shortened. According to the present invention, therefore, the efficiency
of the bulging process is greatly increased.

[0024]As the bulging apparatus has two sets of holding mechanisms, two
workpieces can simultaneously be formed. Consequently, the efficiency of
the bulging process is made much higher.

[0025]Furthermore, there is no need for a mechanism for moving the dies.
Therefore, the apparatus is much simpler in structure.

[0026]A set of holding mechanisms may be employed. Specifically, according
to another aspect of the present invention, a bulging method for
performing a forming process on a hollow member by introducing a fluid
under pressure thereinto, comprises the steps of:

[0027]gripping the hollow member at opposite ends thereof with rods, which
are movable back and forth, of holding mechanisms, with fluid pressure
passages defined in the rods;

[0028]introducing a fluid under pressure into the hollow member gripped by
the rods to expand the hollow member, and stopping the hollow member with
a tube expanding die;

[0029]displacing the holding mechanisms from the tube expanding die to a
main forming die with displacing mechanisms to deliver the expanded
hollow member to the main forming die; and

[0030]forming the hollow member into a product shape with the main forming
die.

[0031]In the above bulging method, since the workpiece gripped by the
holding mechanisms is delivered between the dies, there is no need to
move the dies, and the efficiency of the bulging process is greatly
increased.

[0032]A preforming die may be disposed between the tube expanding die and
the main forming die for preforming the workpiece.

[0033]If the workpiece is preformed, then the hollow member is mainly
formed by the main forming die after the preformed hollow member is
angularly moved. It is thus possible to obtain a finally finished product
having a desired shape.

[0034]The fluid supplied under pressure to the hollow member should
preferably a compressed gas. A compressed gas supply mechanism is smaller
in size than a high-pressure liquid supply mechanism. Therefore, the
investment for facilities is lower, and the installation space is
smaller.

[0035]When the hollow member is preformed or mainly formed, the hollow
member should preferably be supported so as to be translatable in either
a vertical direction or a horizontal direction. The hollow member thus
supported is prevented from being deformed, so that the final formed
product can be produced with excellent dimensional accuracy.

[0036]When the hollow member is heated while being held by the rods, the
hollow member clamped by a heating unit is prevented from flexing by
being pressed by the heating unit.

[0037]According to still another aspect of the present invention, a
bulging apparatus for performing a forming process on a hollow member by
introducing a fluid under pressure thereinto, comprises:

[0038]first holding mechanisms and second holding mechanisms having rods
for gripping the hollow member at opposite ends thereof, the rods being
movable back and forth, with fluid pressure passages defined in the rods;

[0039]a tube expanding die for stopping the hollow member which is gripped
by the rods, while the hollow member is being expanded by a fluid under
pressure being introduced thereinto;

[0040]a preforming die for preforming the expanded hollow member;

[0041]a main forming die for forming the preformed hollow member into a
product shape; and

[0042]displacing mechanisms for displacing the first holding mechanisms
from the tube expanding die to the preforming die or from the preforming
die to the tube expanding die, and displacing the second holding
mechanisms from the preforming die to the main forming die or from the
main forming die to the preforming die;

[0043]wherein the first holding mechanisms and the second holding
mechanisms are displaced by the displacing mechanisms to deliver the
hollow member gripped by the rods of the first holding mechanisms from
the tube expanding die to the preforming die and to deliver the hollow
member gripped by the rods of the second holding mechanisms from the
preforming die to the main forming die.

[0044]As the preforming die and the two sets of holding mechanisms are
provided, two workpieces can simultaneously be formed. Therefore, a final
forming product can efficiently be produced. Stated otherwise, the
efficiency of the bulging process is greatly increased.

[0045]According to yet another aspect of the present invention, a bulging
apparatus for performing a forming process on a hollow member by
introducing a fluid under pressure thereinto, comprising:

[0046]holding mechanisms having rods for gripping the hollow member at
opposite ends thereof, the rods being movable back and forth, with fluid
pressure passages defined in the rods;

[0047]a tube expanding die for stopping the hollow member which is gripped
by the rods, while the hollow member is being expanded by a fluid under
pressure being introduced thereinto;

[0048]a main forming die for forming the expanded hollow member into a
product shape; and

[0049]displacing mechanisms for displacing the holding mechanisms from the
tube expanding die to the main forming die or from the main forming die
to the tube expanding die;

[0050]wherein the holding mechanisms are displaced by the displacing
mechanisms to deliver the hollow member gripped by the rods from the tube
expanding die to the main forming die.

[0051]With the above arrangement, the dies are not moved, but the hollow
member is moved and formed. Therefore, the bulging apparatus is simple in
structure.

[0052]A preforming die may be disposed between the tube expanding die and
the main forming die, for preforming the expanded hollow member. Since
the hollow member can be deformed stepwise, it can be machined more
easily than it is greatly deformed once.

[0053]Either of the above bulging apparatus should preferably have turning
mechanisms for angularly moving the holding mechanisms to make it easy to
produce a final formed product having a desired shape.

[0054]For the reasons described above, the fluid introduced under pressure
into the rods and the hollow member should preferably be a compressed
gas. The bulging apparatus is thus combined with a compressed gas supply
facility. It is preferable to provide a heating unit for heating the
hollow member to be delivered to the tube expanding die in order to
increase the deformability of the hollow tube.

[0055]When the hollow member is formed by the main forming die, the rods
should preferably be translatable in at least one of vertical directions
and horizontal directions, or most preferably be translatable in both
vertical directions and horizontal directions. The hollow member is thus
prevented from being deformed into shapes other than a predetermined
shape, and can be processed into a final formed product with excellent
dimensional accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

[0056]FIG. 1 is a schematic front elevational view of a bulging apparatus
according to a first embodiment;

[0057]FIG. 2 is a schematic side elevational view of the bulging apparatus
shown in FIG. 1;

[0059]FIG. 4 is a schematic view of a holding mechanism of the bulging
apparatus shown in FIG. 1;

[0060]FIG. 5 is a schematic front elevational view of the holding
mechanism shown in FIG. 4;

[0061]FIG. 6 is a schematic view of a plurality of dies of the bulging
apparatus shown in FIG. 1;

[0062]FIG. 7 is a schematic plan view showing the manner in which a first
workpiece is expanded and a second workpiece is disposed in a heating
station;

[0063]FIG. 8 is a schematic vertical cross-sectional view showing the
manner in which a straight tube is expanded by a first lower die and a
first upper die;

[0064]FIG. 9 is a schematic plan view showing the manner in which the
first workpiece that has been expanded is gripped by second holding
mechanisms and the second workpiece is gripped by first holding
mechanisms;

[0065]FIG. 10 is a schematic plan view showing the manner in which the
first workpiece gripped by the second holding mechanisms is being
preformed;

[0066]FIG. 11 is a schematic vertical cross-sectional view showing the
manner in which a first partly finished product is preformed by a second
lower die and a second upper die;

[0067]FIG. 12 is a schematic front elevational view of the holding
mechanisms which have been angularly moved 90° from the position
shown in FIG. 4;

[0068]FIG. 13 is a schematic vertical cross-sectional view showing the
manner in which a second partly finished product is formed in a main
forming process by a third lower die and a third upper die;

[0069]FIG. 14 is a schematic plan view showing the manner in which the
second partly finished product gripped by the second holding mechanisms
is being formed in the main forming process and the second workpiece
gripped by the second holding mechanisms is being expanded;

[0070]FIG. 15 is a schematic plan view showing the manner in which the
first holding mechanisms that have returned to the heating station grip a
third workpiece and the second holding mechanisms that have returned to a
tube expanding station grips the second workpiece; and

[0071]FIG. 16 is a plan view schematically showing, from above, a bulging
apparatus according to a second embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

[0072]Bulging methods according to preferred embodiments will be described
in detail below in relation to forming apparatus for carrying out the
bulging methods with reference to the accompanying drawings.

[0073]FIGS. 1 and 2 are a schematic front elevational view and a schematic
side elevational view, respectively, of a bulging apparatus 10 according
to a first embodiment, and FIG. 3 is a plan view schematically showing,
from above, the bulging apparatus 10.

[0074]As can be understood from FIGS. 1 and 3, the bulging apparatus 10
has a heating station 14 for performing induction heating on a workpiece
12 (hollow member), a tube expanding station 16 for expanding the heated
workpiece 12 by introducing a compressed gas thereinto, a preforming
station 18 for preforming the expanded workpiece 12, and a main forming
station 20 for performing a main forming process on the preformed
workpiece 12. As described later, the workpiece 12 is gripped at its
opposite ends by a set of first holding mechanisms 22a, 22a and fed
between the stations 14, 16, and gripped at its opposite ends by a set of
second holding mechanisms 22b, 22b and fed between the stations 18, 20.

[0075]Heating electrodes 24 (see FIG. 3) as heating units are disposed in
the heating station 14, and a tube expanding die 26 for expanding the
workpiece, a preforming die 28 for preforming the workpiece, and a main
forming die 30 for performing the main forming process are disposed
respectively in the tube expanding station 16, the preforming station 18,
and the main forming station 20 (see FIG. 1).

[0076]The bulging apparatus 10 has a main frame 32, and includes a lower
auxiliary frame 34 and an upper auxiliary frame 36 which extend from the
heating station 14 to the main forming station 20. As described later,
the first and second holding mechanisms 22a, 22b are displaceable along
the lower auxiliary frame 34 and the upper auxiliary frame 36.

[0077]The workpiece 12 gripped by the first holding mechanisms 22a, 22a in
the heating station 14 is delivered to the tube expanding station 16, the
preforming station 18, and the main forming station 20 as the first
holding mechanisms 22a, 22a and the second holding mechanisms 22b, 22b
which subsequently grip the workpiece 12 are displaced successively to
the tube expanding die 26, the preforming die 28, and the main forming
die 30.

[0078]As shown in FIG. 4, each of the first holding mechanisms 22a has a
body 38 extending from an open end of a holder 40 into a hole 42 defined
in the other end thereof. The holder 40 has a lower end surface and an
upper side surface on which a first engaging member 44 and a side plate
46 are mounted, respectively, with a second engaging member 48 being
mounted on the side plate 46. The first engaging member 44 and the second
engaging member 48 are held in slidable engagement with guide rails 50,
52, respectively, that are laid on the lower auxiliary frame 34 and the
upper auxiliary frame 36, respectively.

[0079]A servomotor 54 serving as a displacing mechanism is fixedly mounted
on the side plate 46. As shown in FIGS. 1 and 5, the servomotor 54 has a
rotational shaft 56 over which a first pinion 58 is fitted. A first rack
60 is mounted on an upper end surface of the upper auxiliary frame 36,
and the first pinion 58 is held in mesh with the first rack 60.

[0080]As shown in FIG. 4, a workpiece gripping cylinder 62 is mounted on
an end of the body 38. The workpiece gripping cylinder 62 has a rod 64
projecting from the hole 42 defined in the other end of the body 38 and
movable back and forth in the directions indicated by the arrows X1, X2
in FIG. 4. The rod 64 has a compressed air passage, not shown, defined
therein.

[0081]The body 38 is angularly movable 90° by a body turning
cylinder 104 serving as a turning mechanism. As shown in FIGS. 1, 4, and
5, the body turning cylinder 104 is disposed beneath the body 38 and has
a rod 106 to which there is coupled a driven rod 110 extending parallel
to the body turning cylinder 104 by a coupling member 108. A second rack
112 is mounted on the driven rod 110. An arcuate second pinion 116 held
in mesh with the second rack 112 is fixedly mounted on a peripheral side
wall of a tubular member 114 of the body 38. When the rod 106 of the body
turning cylinder 104 is moved back and forth in the directions indicated
by the arrows Y1, Y2 in FIG. 5, the tubular member 114 and the body 38
are angularly moved in the directions indicated by the arrows B1, B2.

[0082]A bracket 118 is mounted on an upper portion of the body 38 (see
FIG. 4), and a rod 122 of a body lifting/lowering cylinder 120 serving as
a lifting/lowering mechanism has a distal end housed in the bracket 118
with play. Specifically, the distal end of the rod 122 is so wide that it
is prevented from being removed from the bracket 118. The rod 122 lifts
and lowers the body 38 indirectly through the bracket 118.

[0083]The body lifting/lowering cylinder 120 is mounted on an upper end
surface of the holder 40, and the rod 122 extends through a passage hole
124 defined in the upper end surface of the holder 40. Stated otherwise,
the body lifting/lowering cylinder 120 is inverted and fixedly positioned
on the upper end surface of the holder 40.

[0084]Each of the second holding mechanisms 22b is of an identical
construction (see FIG. 2). Therefore, identical parts are denoted by
identical reference characters, and will not be described in detail
below.

[0085]The heating electrodes 24 shown in FIG. 3 are disposed in the
heating station 14. Each of the heating electrodes 24 is movable toward
and away from the workpiece 12. The lower heating electrodes 24 and the
upper heating electrodes 24 are disposed in positions confronting each
other across the workpiece 12, so that the workpiece 12 is heated in its
entirety by the four heating electrodes 24.

[0086]The tube expanding station 16, the preforming station 18, and the
main forming station 20, which are disposed parallel to the heating
station 14, have the tube expanding die 26 for expanding the workpiece,
the preforming die 28 for preforming the workpiece, and the main forming
die 30 for performing the main forming process, disposed respectively
therein (see FIG. 3). As shown in FIGS. 2 and 6, the tube expanding die
26 has a first lower die 130 and a first upper die 132. A first lower die
cylinder 134 is supported on the main frame 32 and has a rod connected to
the first lower die 130 (see FIG. 1). The first lower die 130 is thus
vertically displaceable by the first lower die cylinder 134.

[0087]The preforming die 28 in the preforming station 18 and the main
forming die 30 in the main forming station 20 are constructed essentially
identically to the tube expanding die 26. Specifically, as shown in FIGS.
1 and 6, the preforming die 28 has a second lower die 180 and a second
upper die 182. A second lower die cylinder 184 is supported on the main
frame 32 and has a rod connected to the second lower die 180. Similarly,
the main forming die 30 has a third lower die 188 and a third upper die
190. A third lower die cylinder 192 has a rod connected to the third
lower die 188. The second lower die 180 and the third lower die 188 are
vertically displaceable by the cylinders 184, 192 (see FIG. 1).

[0089]The bulging apparatus 10 according to the first embodiment is
basically constructed as described above. Operation and advantages of the
bulging apparatus 10 will be described below with respect to a bulging
process for forming a straight tube of aluminum alloy. In the description
which follows, the straight tube will be denoted by the reference
character 12 that has been used to denote the workpiece.

[0090]As shown in FIG. 3, for example, the first and second holding
mechanisms 22a, 22b are disposed respectively in the heating station 14
and the tube expanding station 16. After the straight tube 12 is
delivered to the heating station 14, the straight tube 12 is gripped by
the first holding mechanisms 22a, 22a.

[0091]The heating electrodes 24 (see FIG. 3) are energized for heating and
moved toward the straight tube 12. Finally, the heating electrodes 14
have their distal ends brought into abutment against the peripheral side
wall of the straight tube 12 to heat the straight tube 12.

[0092]The straight tube 12 is thermally expanded as it is heated to about
450° to 550° C. That is, the longitudinal dimension of the
straight tube 12 becomes large. According to the first embodiment, the
distal ends of the heating electrodes 14 are held in abutment against the
straight tube 12 while the heating electrodes 24 are displaceable in the
longitudinal directions of the straight tube 12. Therefore, as the
straight tube 12 is longitudinally elongated by thermal expansion, the
heating electrodes 24 are displaced along the longitudinal directions of
the straight tube 12. Consequently, even when the straight tube 12 is
thermally expanded, the regions of the straight tube 12 that are clamped
by the heating electrodes 24 are prevented from buckling.

[0093]The straight tube 12 thus heated is delivered to the tube expanding
station 16 (see FIGS. 1 and 3) when the first holding mechanisms 22a are
displaced. Specifically, the body lifting/lowering cylinders 120 (see
FIG. 4) are actuated to retract the rods 122 thereof. As a result, the
brackets 118 are pulled by the rods 122, lifting the bodies 38.

[0094]Then, the servomotors 54 are energized to start rotating the
rotational shafts 56. The first pinions 58 which are rotated by the
rotational shafts 56 roll in mesh with the first racks 60, and, as a
result, the first holding mechanisms 22a start being displaced toward the
tube expanding station 16. At this time, the first holding mechanisms 22a
are guided by the guide rails 50, 52 mounted on the lower auxiliary
frames 34 and the upper auxiliary frames 36.

[0095]When the first holding mechanisms 22a reach the tube expanding
station 16, the servomotors 54 are de-energized, and the first pinions 58
stop rolling in mesh with the first racks 60 and the first holding
mechanisms 22a stop being displaced. The body lifting/lowering cylinders
120 are actuated to expand the rods 122 to lower the first holding
mechanisms 22a until finally the straight tube 12 is positioned between
the first lower die 130 and the first upper die 132. The first lower die
130 and the first upper die 132 have been heated to a predetermined
temperature by the heating means, not shown.

[0096]As the first holding mechanisms 22a are displaced to the tube
expanding station 16, the second holding mechanisms 22b disposed in the
tube expanding station 16 are displaced to the preforming station 18 (see
FIG. 7). On the other hand, a second straight tube 12 is provided in the
heating station 14.

[0097]Thereafter, the first lower die cylinder 134 is actuated to lift the
first lower die 130 toward the straight tube 12. The die is closed, and,
as shown in FIG. 8, the straight tube 12 is placed in a cavity 196
defined by the first lower die 130 and the first upper die 132. The
cavity 196 have some dimensions greater than the straight tube 12 such
that the peripheral side wall of the straight tube 12 has portions spaced
from both the first lower die 130 and the first upper die 132.

[0098]Then, compressed air is supplied through the compressed air passages
in the rods 64 into the straight tube 12, developing a pressure buildup
in the straight tube 12. Specifically, the straight tube 12 is pressed
from inside thereof by the compressed air, forcing the portions of the
straight tube 12 which are spaced from the first lower die 130 and the
first upper die 132 to start expanding toward the first lower die 130 and
the first upper die 132.

[0099]The expanded portions are finally stopped by the first lower die 130
and the first upper die 132. The expansion is stopped, thereby forming a
first partly finished product 200 that is shaped complementarily to the
cavity 196, as shown in FIG. 7.

[0100]While the straight tube 12 is being thus expanded, since the first
lower die 130 and the first upper die 132 is being heated by the heating
means, the temperature of the straight tube 12 is prevented from being
lowered.

[0101]If the straight tube 12 is expanded without its longitudinal
dimension remaining unchanged, then the wall thickness of the straight
tube 12 is reduced. In order to prevent the wall thickness of the
straight tube 12 from being reduced, the rods 64 are moved forward as the
straight tube 12 is expanded. Therefore, the wall thickness of the first
partly finished product 200 is not reduced.

[0102]Upon elapse of a predetermined time after the die is closed, the
compressed air is discharged through the compressed air passages in the
rods 64. The first lower die 130 is lowered to open the die, as shown in
FIG. 7.

[0103]When the tube expanding process is finished, the first holding
mechanisms 22a, 22a are retracted to release the first partly finished
product 200. As shown in FIG. 9, the first holding mechanisms 22a, 22a
returns to the heating station 14 and grip the second straight tube 12,
and the second holding mechanisms 22b, 22b return to the tube expanding
station 16 and grip the first partly finished product 200 (formed from
the first straight tube 12).

[0104]The first partly finished product 200 is delivered to the preforming
station 18 in the same manner as the straight tube 12 is delivered from
the heating station 14 to the tube expanding station 16 (see FIG. 10).
Specifically, the body lifting/lowering cylinders 120 (see FIG. 4) of the
second holding mechanisms 22b are actuated to lift the bodies 38, and
then the servomotors 54 are energized to cause the first pinions 58 to
roll on the first racks 60. The second holding mechanisms 22b are
displaced toward the preforming station 18. At this time, the second
holding mechanisms 22b are also guided by the guide rails 50, 52.

[0105]When the first partly finished product 200 is thus delivered, it is
disposed between the second lower die 180 and the second upper die 182.
Thereafter, the servomotors of the body lifting/lowering cylinders 120
are de-energized, bringing the body lifting/lowering cylinders 120 into a
so-called servo-free state. When an external force is applied to the rods
122 of the body lifting/lowering cylinders 120, the rods 122 are moved
back and forth by a displacement depending on the magnitude of the
external force.

[0106]Then, only the second lower die cylinder 184 is actuated to elevate
the second lower die 180 to press the first partly finished product 200
toward the second upper die 182.

[0107]Since the body lifting/lowering cylinders 120 are held in the
servo-free state, the rods 64 of the second holding mechanisms 22b are
lifted parallel to the vertical direction as indicated by the imaginary
lines in FIG. 4 as the first partly finished product 200 is pressed by
the second lower die 180.

[0108]As can be seen from the foregoing, even when the first partly
finished product 200 is pressed upwardly, it receives no resistance from
the body lifting/lowering cylinders 120. Therefore, the first partly
finished product 200 can easily be brought closely to the second upper
die 182. As the second holding mechanisms 22b are vertically
displaceable, the pressed first partly finished product 200 is not
released from the second holding mechanisms 22b. Consequently, no
compressed air leaks from between the first partly finished product 200
and the second holding mechanisms 22b.

[0109]Immediately before the second lower die 180 abuts against the first
partly finished product 200, compressed air is supplied from the
compressed air passages in the rods 64. The pressure under which the
compressed air is supplied may be set to a level not large enough to
expand the first partly finished product 200.

[0110]When the second lower die 180 abuts against the first partly
finished product 200, a cavity 202 is defined as shown in FIG. 11. The
first partly finished product 200 is partly squeezed into an elliptical
cross-sectional shape, thereby producing a second partly finished product
204 shown in FIG. 10.

[0111]The squeezing (preforming) progresses as the second lower die 180
approaches the second upper die 182. During the squeezing, the
servomotors 54 are in a servo-free state, i.e., the forces on the
rotational shafts 56 of the servomotors 54 are reduced. Therefore, during
the squeezing, the second holding mechanisms 22b are translatable in the
direction (horizontal direction) in which the stations 14, 16, 18 are
juxtaposed.

[0112]When the squeezing is finished, the compressed air is discharged,
and the second lower die cylinder 184 is actuated to lower the second
lower die 180. The die is opened, and since the body lifting/lowering
cylinders 120 are held in the servo-free state, the second partly
finished product 204 returns to its initial position.

[0113]While the preforming process is thus being performed to produce the
second partly finished product 204 in the preforming station 18, the
second straight tube 12 may continuously be heated in the heating station
14, as shown in FIG. 10.

[0114]The exposed second partly finished product 204 is then delivered to
the main forming station 20 in the same manner as the first partly
finished product 200 is delivered from the tube expanding station 16 to
the preforming station 18. Specifically, the body lifting/lowering
cylinders 120 (see FIG. 4) of the second holding mechanisms 22b are
actuated to elevate the bodies 38 in the same manner as described above.
Then, the servomotors 54 are energized to cause the first pinions 58 to
roll on the first racks 60. The second holding mechanisms 22b are
displaced toward the main forming station 20 while being guided on the
guide rails 50, 52.

[0115]During the delivery of the second partly finished product 204, the
body turning cylinders 104 (see FIG. 5) are actuated. As shown in FIG.
12, when the rods 106 are retracted in the direction indicated by the
arrow Y2, the driven rods 110 are retracted, and the second pinions 116
held in mesh with the second racks 112 are rotated 90° in the
direction indicated by the arrow B2. Then, the body 38 and the holding
mechanism 22 are rotated 90° in the direction indicated by the
arrow B2. The second partly finished product 204 is also angularly moved
90°. Specifically, upon completion of the preforming, the second
partly finished product 204 includes a portion which is horizontally
elongate elliptical cross-sectional shape (see FIG. 11). As the bodies 38
are rotated, the portion of the second partly finished product 204
becomes vertically elongate in its elliptical cross-sectional shape.

[0116]When the delivery of the second partly finished product 204 is
finished, the second partly finished product 204 is placed between the
third lower die 188 and the third upper die 190. Thereafter, the body
lifting/lowering cylinders 120 are brought into a servo-free state in the
same manner as described above.

[0117]Then, the third lower die cylinder 192 is actuated to lift the third
lower die 188 to press the second partly finished product 204 toward the
third upper die 190.

[0118]At this time, since the body lifting/lowering cylinders 120 are held
in the servo-free state as with the preforming process, the rods 64 of
the second holding mechanisms 22b are lifted parallel to the vertical
direction as indicated by the imaginary lines in FIG. 4 as the second
partly finished product 204 is pressed by the third lower die 188.

[0119]Consequently, the second partly finished product 204 can easily be
brought closely to the third upper die 190. The second partly finished
product 204 which is pressed by the third lower die 188 is not released
from the rods 64.

[0120]Immediately before the third lower die 188 abuts against the second
partly finished product 204, compressed air is supplied from the
compressed air passages in the rods 64.

[0121]When the third lower die 188 abuts against the second partly
finished product 204, a cavity 206 is defined as shown in FIG. 13. The
second partly finished product 204 is partly squeezed, thereby producing
a final formed product 208 shown in FIG. 14.

[0122]As can be understood from FIG. 13, the pressing force applied in the
main forming process is smaller for pressing the vertically elongate
elliptical shape than for pressing the horizontally elongate elliptical
shape. The pressing force applied in the main forming process can thus be
reduced by turning the second partly finished product. Since the
horizontal dimension of the second partly finished product 204 is small,
the width of the main forming die 30 which corresponds to the horizontal
dimension of the second partly finished product 204 may be reduced.

[0123]In the main forming process, the second holding mechanisms 22b are
horizontally movable as the second partly finished product 204 is formed
if the servomotors 54 are held in a servo-free state.

[0124]When the main forming process is finished, the compressed air is
discharged, and the workpiece gripping cylinders 62 are actuated.
Specifically, the rods 64 are retracted in the direction indicated by the
arrow X2 in FIG. 4, releasing the final formed product 208 from the
second holding mechanisms 22b, 22b onto the third lower die 188.

[0125]Thereafter, the third lower die cylinder 192 is actuated to lower
the third lower die 188 with the final formed product 208 placed thereon.

[0126]The final formed product 208 placed on the third lower die 188 is
gripped by a robot having a take-out jig. An ejector on the third lower
die 188 is actuated to release the final formed product 208 from the
third lower die 188. The released final formed product 208 is taken out
by the robot, and then fed to a next process.

[0127]At the same time that the second partly finished product 204 is
delivered to the main forming station 20 by the displacement of the
second holding mechanisms 22b, the straight tube 12 is delivered to the
tube expanding station 16 by the displacement of the first holding
mechanisms 22a, as shown in FIG. 14. Stated otherwise, at the same time
that the main forming process is performed on the second partly finished
product 204, the tube expanding process is performed on the straight tube
12. A third straight tube 12 is provided in the heating station 14.

[0128]When the main forming process on the second partly finished product
204 is finished, as mentioned above, the second holding mechanisms 22b
release the final formed product 208 (the first straight tube 12), and
the first holding mechanisms 22a release the first partly finished
product 200 (the second straight tube 12). As shown in FIG. 15, the final
formed product 208 is ejected, and the first holding mechanisms 22a, 22a
return to the heating station 14 to grip the third straight tube 12, and
the second holding mechanisms 22b, 22b return to the tube expanding
station 16 to grip the first partly finished product 200. Subsequently, a
procedure similar to the above procedure will be repeated.

[0129]According to the present embodiment, the straight tube 12 as a
workpiece is held by the first holding mechanisms 22a, 22a or the second
holding mechanisms 22b, 22b, and delivered between the dies 26, 28, 30.
Therefore, the dies 26, 28, 30 do not need to be moved.

[0130]According to the first embodiment, the dies 26, 28, 30 are
juxtaposed. Therefore, the apparatus is not complex in structure, and the
dies 26, 28, 30 do not need to be movably installed. As there is no need
to move the dies 26, 28, 30 which are heavy, the cycle time is shortened.

[0131]The first embodiment is advantageous in that since the apparatus is
constructed to deliver the workpiece 12 between the dies 26, 28, 30, the
apparatus is structurally simpler, operates more simply, and has a
shorter cycle time until the final formed product 208 is obtained, than
if the workpiece 12 is delivered between the dies 26, 28, 30 by a robot.

[0132]According to the first embodiment, furthermore, two straight tubes
12, 12 can simultaneously be formed. Therefore, the cycle time is
shortened.

[0133]A bulging apparatus according to a second embodiment will be
described below. FIG. 16 is a plan view schematically showing, from
above, a bulging apparatus 210 according to a second embodiment.

[0134]The bulging apparatus 210 is constructed in accordance with the
bulging apparatus 10 according to the first embodiment except that it has
a set of holding mechanisms 212 which are identical in structure to the
first and second holding mechanisms 22a, 22b. Various components are
constructed and operate in the same manner as with the first embodiment.

[0135]In the bulging apparatus 210 according to the second embodiment, the
holding mechanisms 212, 212 grip a workpiece and are displaced between
the stations 14, 16, 18, 20. Specifically, a straight tube 12 is first
gripped by the holding mechanisms 212, 212 in the heating station 14 and
heated by the heating electrodes 24. The holding mechanisms 212 are
displaced to the tube expanding station 16 in the same manner as with the
first embodiment to deliver the straight tube 12 to the expanding station
16.

[0136]Then, the straight tube 12 is expanded in the tube expanding station
16, producing a first partly finished product 202. The first partly
finished product 202 is delivered to the preforming station 18 when the
holding mechanisms 212 are displaced to the preforming station 18.

[0137]The first partly finished product 202 is preformed in the preforming
station 18, producing a second partly finished product 204. The second
partly finished product 204 is delivered to the main forming station 20
when the holding mechanisms 212 are displaced to the main forming station
20. Finally, the main forming process is performed on the second partly
finished product 204 in the main forming station 20, producing a final
formed product 208.

[0138]As described above, the workpiece can be delivered between the dies
26, 28, 30 by only the single set of holding mechanisms 212, 212.

[0139]In the above embodiments, compressed air is introduced into the
straight tube 12. The workpiece is not limited to the straight tube 12
whose cross-sectional shape is circular, but may be a hollow member whose
cross-sectional shape is polygonal.

[0140]In each of the first and second embodiments, the bulging apparatus
has the heating station 14, the tube expanding station 16, the preforming
station 18, and the main forming station 20. However, both the bulging
apparatus 10, 210 may be arranged so as to be free of the heating station
14.

[0141]In the second embodiment, the tube expanding station 16 and the main
forming station 20 may make up a bulging apparatus. If necessary, the
heating station 14 may be added.

[0142]At any rate, the holding mechanisms 22 may be translated in at least
one of the horizontal direction and the vertical direction.